The present invention relates to photosensitive imaging systems and, more particularly to a method of imaging photosensitive imaging media to improve its sensitometric response by increasing the media temperature during the period of time (xe2x80x9cdark time stagexe2x80x9d) between image-wise exposure of the media and pressure development. While the method of the present invention is useful in improving imaging characteristics of photosensitive imaging media generally, it is believed to be particularly useful in improving the stability of the middle tone and toe sensitivities of the HandD curve of self-contained photosensitive media employing photosensitive microcapsules. It is believed that a photosensitive imaging media imaged in accordance with the present invention is less sensitive to changes in environmental conditions, particularly changes in humidity, and exhibits improved image retention with time.
Photohardenable imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209; 4,416,966; 4,440,846; 4,766,050; 4,962,010 and 5,283,015. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photohardenable composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers. An image transfer system in which the developer material is coated on a separate substrate as a separate developer or copy sheet is disclosed in commonly assigned U.S. Pat. No. 4,399,209. A self-contained imaging sheet in which the encapsulated color former and the developer material are co-deposited on one surface of a single substrate as one layer or as two interactive layers is disclosed in commonly assigned U.S. Pat. No. 4,440,846. An imaging system comprising a support, a layer containing microcapsules, a layer of developer material, and a layer containing an opacifying agent is disclosed in commonly assigned U.S. Pat. No. 4,766,050. The opacifying agent can form a separate layer or can be part of the layer containing the microcapsules or both but is interposed between the microcapsules and the developer to hide the microcapsules when viewing the image.
U.S. Pat. No. 5,783,353, commonly assigned, discloses a self-contained imaging system wherein the imaging layer is sealed between two supports to form an integral unit. The laminated format is advantageous in that it can reduce oxygen and moisture permeation and improve stability of the media. U.S. patent application Ser. No., filed Jan. 16, 2001, discloses a self-contained photosensitive material which includes an imaging layer of photosensitive microcapsules and a developer on a support and a protective coating on the imaging layer.
U.S. Pat. No. 4,873,168 to Ruder, et al. discloses a non-enclosed image-forming system employing photosensitive microcapsules which are heat treated prior to developing. Ruder discloses heating the imaging sheet for a time period from 30 seconds to as long as 20 minutes to extend the dynamic range of the color forming. By contrast, the present invention relates to increasing the temperature of an enclosed imaging media during the dark time stage which effectively reduces the dynamic range. Accordingly, the structure of the photosensitive imaging media described in Ruder is different (open versus enclosed) and the impact of heating the photosensitive media on the sensitometric response is different as well.
U.S. Pat. No. 6,077,810 to Imaeda discloses heating a photosensitive print medium after pressure developing to improve toner qualities. Imaeda is directed solely to post-heating the image media after pressure development; there is no disclosure or discussion of heating the media during the dark time stage.
In accordance with the present invention, a method of imaging photosensitive imaging media to improve its sensitometric response by increasing the media temperature during the period of time (xe2x80x9cdark time stagexe2x80x9d) between image-wise exposure of the media and pressure development is disclosed. In addition, the photosensitive imaging media which are prepared in accordance with the present invention exhibit improved image retention. Further, by utilizing the method of the present invention, humidity sensitivity of the photosensitive imaging media is reduced thereby producing more consistent and reliable images. The method of the present invention involves increasing the temperature of the imaging media during the dark time stage. This promotes better control of dye release, even under high speed printing conditions. This may be due to better crosslinking of the photohardenable polymer or to improved resistance of the microcapsule wall.
In one embodiment of the present invention, the method of imaging a photosensitive media to improve its sensitometric response includes the steps of providing a photosensitive imaging media including an imaging layer comprising a developer material and a plurality of microcapsules encapsulating a photohardenable composition and a color former; image-wise exposing the imaging media to actinic radiation to form a latent image on the media; heating the imaging media containing the latent image; and pressure developing the latent image.
In accordance with a particular embodiment of the present invention, the imaging media is heated to a temperature of above about 50xc2x0 C. At temperatures above 80xc2x0 C., the effect does not improve substantially. Accordingly, while higher temperatures can be used, there is generally not a need to use them. The imaging media may be heated for a heating time of from about 1 millisecond to 30 seconds depending upon the temperature and efficiency of the heating element. In some embodiments of the present invention, the imaging media is first heated and then allowed to cool before development.
In accordance with another embodiment of the present invention, the imaging media is heated before or during exposure and maintained at an elevated temperature both during exposure and the dark time stage. Again, it may be advantageous to allow the imaging media to equilibrate to a lower temperature prior to pressure development.
Dark time heating in accordance with the present invention is believed to be particularly useful in improving the sensitometric response of enclosed imaging media. Enclosed media refer to systems constructed either as a laminated self-contained sheet between two films or as a one base self contained sheet including a protective coating. These types of imaging systems when imaged in accordance with the present invention exhibit more consistent and more predictable photoimaging characteristics under varying environmental conditions. For example, sensitivity of the photosensitive media to changes in humidity from, e.g., 15% to 80% RH is reduced by dark time heating such that consistent print quality can be obtained over a range of humidity conditions.